‘The last piece in the jigsaw” is how architect Eric Parry described his design for 1 Undershaft, the tallest building in the City of London, when it was unveiled this week. It will crown the planned “cluster” of towers, standing right between the Cheesegrater and the Gherkin. But it’s far from the final piece – in fact, it’s only the beginning of another building boom.

In an anonymous basement a few streets from the 1 Undershaft site stands a chessboard showing the City of London’s future skyline. ThePerspexpeaks cluttering this planning office model glowgreenish-grey, like a malevolent crystal formation from the planet Krypton. Dreamed up during the banking bubble, most of these novelty chess pieces were stopped dead by the recession. But now, with the global property investment industry booming and the City’s vacancy rates at their lowest for 15 years, the tabletop fantasy is fast becoming reality.

Fly through the future

North of the Gherkin, a tower nicknamed the Can of Ham for its odd tubby form is currently being built. Designed seven years ago by Foggo Associates, the 24-storey spam tin has been revived by one of the world’s biggest pension funds, TIAA-CREF. Meanwhile, cranes are conjuring the 38-storey Scalpel – a stunted cousin of the Shard by US architects Kohn Pederson Fox. “The Skyline Refined” trumpets its construction hoarding, showing the glass blade reaching to the skies, as if poised to slice up its neighbours.

Which might not be a bad thing, given what’s coming next door. The gargantuan staggered slabs of Make’s 40 Leadenhall Street, for Henderson Global Investors, will form a building more than twice the Scalpel’s bulk. Dubbed Gotham City, for the sense of dystopian gloom it will cast over all who see it, it has the unfortunate quality of being a groundscraper and a skyscraper at the same time – and resembling a corpulent grey heap.

Some of the biggest pieces are still missing from the chessboard, as the planning office struggles to keep up with what developers are throwing at it. 22 Bishopsgate was recently granted permission on the site of the ill-fated Pinnacle, where a seven-storey concrete stump has stood since construction stalled in 2008 – a mocking relic of the last banking bubble. It will combine the total internal area of the Cheesegrater and the Walkie-Talkie inside one slab that’s 80m-wide and 278m high – enough space for 12,000 people.

22 Bishopsgate was fiercely opposed by the neighbouring boroughs of Islington and Tower Hamlets, along with Royal Parks and Historic Royal Palaces, who feared its “broad-shouldered” profile would create a “solid backdrop” to the Tower of London, compared to the previous scheme’s tapered peak. Even Heathrow, 15 miles away, complained that its heft would interfere with radar coverage. Permission was granted nonetheless.

The shift from the spiralling helter-skelter of the Pinnacle to this squarer, fatter wodge not only reflects the desire of the project’s new backer, Axa Real Estate, to squeeze 30% more volume out of the £300m they paid for the site, but also reveals the changing priorities of the City under its new director of design.

“The new scheme follows our guidance for sleek simplicity, in contrast to some of the more gimmicky buildings we’ve had recently,” says Gwyn Richards, who was appointed head of design at the City of London’s planning team last year following the departure of Peter Rees. “We’re taking a more disciplined approach now: no building in the cluster should be trying to shout down its neighbour.”

Richards is frank. He describes the current City skyline as an “incoherent riot”, particularly when seen from cherished vantage points such as Westminster Bridge. “It would be fair to say that, over time, we fumbled into the cluster,” he adds. “Now it’s time for clarity on the skyline.”

Looming 160m above Fenchurch Street, towering over several conservation areas and butting into the background of most views of London, the Walkie-Talkie is perhaps the most egregious example of such incoherence. Built in 2010, the overbearing tower is widely regarded to have destroyed any semblance of planning logic the City ever had . It makes a mockery of trying to control tall buildings in the City, says one conservation campaigner: “It’s like trying to pour water in a neat pile, then being surprised when it spreads.”

Rees, to extend the metaphor, wielded the watering can over the City from 1985 to 2014. As the man who enthusiastically ushered the Carbuncle Cup-winning Walkie-Talkie through the system, he begs to differ. “I saw the building as the figurehead at the prow of our ship,” he says. “The ‘sky garden’ is a place to look back at the engine room of the City behind you, a wonderful grandstand to admire the architecture of the last two decades. If we were going to build a tower outside the cluster, it had to be a distinct object which could stand alone on the skyline. I’m very proud of it – and it’s fully let.” Quite how long it stands there alone remains to be seen.

Like much of London’s development since the 1980s, the idea of locating the tallest buildings in the City emerged from a combination of “haphazard chance and rigid controls”, says Richards. The cluster exists in the area left over once conservation areas and listed buildings are subtracted – along with the web of ley lines that protect the hallowed views of St Paul’s cathedral and the Tower of London, plus hundreds of other significant townscape views. It is a fiendishly complex cat’s cradle of invisible planes beneath which all buildings must limbo.

These rigid rules first appeared in the 1930s, when the surveyor of St Paul’s came up with a precise grid of heights around the cathedral to safeguard the panorama from the South Bank. “We’ve been able to calculate where he took his views from, and most of them end up outside pubs,” says Rees.

But the blurry glimpse of Wren’s dome he might have caught when stumbling out of a Southwark tavern still defines what can be built between the cathedral and the river. That grid is now manifest in stone and steel: all the buildings between St Paul’s and the Thames have a stepped roofline. “Some people say it looks dreadful,” says Rees, “and I tend to agree”.

All lines lead to St Paul’s

Regulations to safeguard long-distance views come in the form of the London View Management Framework. Governed by the mayor, this outlines a number of protected corridors from places such as Hampstead Heath, Alexandra Palace and King Henry’s mound in Richmond Park, where a special hole is still cut in the hedge to preserve the view of St Paul’s – so far away that planning calculations must incorporate the curvature of the earth.

All these invisible lines lead to St Paul’s, which stands sentry, keeping watch over the metropolis it has shaped for 300 years, sometimes in highly specific ways. The “dynamic view” of St Paul’s, travelling eastwards down Fleet Street to the west of the cathedral, has probably given more of the recent towers their shape than any other constraint or architect’s whim.

How Fleet Street shaped the cluster

The Cheesegrater leans back to the north specifically to dodge this view, a gymnastic feat that took more than twice as much steel as the Eiffel Tower to achieve. And its lean set a precedent. To the south, the Scalpel has been designed to slope back in a mirror-image incline, forming an awkward architectural salsa dance over the road. Its glacial facade has been sliced away specifically to preserve blue sky at the other side of St Paul’s dome, the two buildings together forming a “valley” when seen from Fleet Street. And its pointy peak? That came from how it looks from Waterloo Bridge.

“It was originally much squatter, with a very apologetic top, forming a flat profile on the skyline,” says Richards. “I suggested they go higher – we wanted to create a dynamic feeling which reinforces the slope up to the top of the cluster. I don’t think they were very happy doing it because they didn’t have additional floorspace.” Then he adds: “We lower buildings sometimes, we push them up other times. But there is certainly no testosterone urge.”

A new tower planned to the west of the Cheesegrater also had to follow its lean. 6-8 Bishopsgate, an elegant 40-storey stack of simple glass boxes by Wilkinson Eyre, will step back to the north to hide within the shadow of Rogers’ tapered wedge. “We were very keen to preserve the silhouette of the Leadenhall building [Cheesegrater],” says Richards. It’s widely recognised as successful.” A proposal using the same logic was made for the Undershaft site, but it took the “hiding in the shadow” principle a step too far, mimicking the building’s profile like some fanboy stalker.

Like 22 Bishopsgate, Wilkinson Eyre’s tower mirrors the City’s new preference for “quieter” architecture. The project started out by trying to ape its neighbours: the first concept looked like the lovechild of the swirling Pinnacle and the tilting Cheesegrater. Combined, the trio would have made the cluster look like a bucket of rolled-up newspapers, and it was duly tamed. “The new regime thought there were too many curved shapes that didn’t fit together,” says architect Chris Wilkinson.

Their tower will have a small viewing gallery at the very top, identified by a separate box, following the City’s guidance that the next generation of sky gardens must be recognisable from the street. “It used to be that you would look up at these financial citadels and imagine the view from the boardroom,” says Richards, explaining the new policy for every tower to have free public access up high. “Now we want people to feel ‘I own that building.’ It’s for Londoners to take over the City again.”

Gardens in the sky

City gamekeeper-turned-poacher Peter Rees has become an unlikely critic of tall buildings since he left office last year after almost three decades as chief planner. “I’ve always said that building tall is the last resort,” he says, sitting in the senior common room of University College London, where he is now professor of places and planning. “You should only do it once you’ve run out of space and you need to maintain the vitality of an already successful place.” (In his eyes, the outcrop of towers springing up elsewhere in London – along the river from Bermondsey to Battersea, and at City Road and Aldgate – are a different matter. “These are silos of safety deposit boxes. They are doing nothing for London but fuelling the overseas investment market.”)

As the man who gave permission to most of these towers, he kept policies for tall buildings intentionally vague. “British planning is reactive,” he explains. “We don’t have the masterplanning that you find in most parts of the world, so you have to respond to whatever developments come along and give permissions not knowing whether they will ultimately be built or not. We can’t enforce the construction of what’s been approved.” The cluster, he says, must be a “flexible broad-brush concept” to respond to the market, given that the City is the UK’s “engine room for growth”.

One of the few regulations that has been spelt out in black and white is the maximum height limit – so planes don’t have to weave between spires on their way to and from City Airport, five miles to the east. The Civil Aviation Authority has set a “safeguarded surface” of 305m across the Square Mile, determined by the fact that planes must have a 1,000 ft clearance above any building.

Watch that plane

This throws 1 Undershaft’s proposed height of 309.6m (the same height as the Shard) into serious doubt. Yet developers have always argued that the pointed top of One Canada Square sets a precedent for going higher in the City: planes have begun their descent by the time they fly over Canary Wharf, so they only clear the tower by 600 feet (183m). “If someone came along with a taller proposal we would certainly consider it,” says Richards.

But why not set out clear rules for towers in the City and stick to them? “The minute you map these things, you’re in real trouble,” Rees says. “Developers go into a feeding frenzy. As soon as you define what the biggest possible development could be on each site, developers will come and build it. Before you know it, everything is built out to its maximum bulk. It’s like blowing a whistle when you have a pack of hungry dogs nearby.”

The consequences of landowners pushing to the limits can be clearly seen at the choked centre of the cluster, where sites are being filled to their boundaries and inflated until they crash into something. The southern wall of 22 Bishopsgate will stand just three metres from the north face of 6–8 Bishopsgate, creating a narrow canyon 40-storeys tall. Seeing these buildings together in a model, it is incomprehensible that such a skyline could ever be green-lit – let alone physically built.

Too close for comfort

Besides blocking the sky, there will be consequences on the ground. After the Walkie-Talkie was found to create gusts of wind that can sweep people off their feet, it raises questions about the effect this new wall of giants standing shoulder-to-shoulder will have on just being able to walk down the street.

Eric Parry has identified an area for an “artist’s wind-baffle commission” at the most hazardous spot on the 1 Undershaft plaza, while Richards assures that wind issues are being taken more seriously. He, however, is a fan of the buildings being so close. “There’s a real psychology of proximity in the cluster,” says Richards. “The insurance industry, more than anything else, relies on face-to-face contact. There’s a culture of huddling together for collective benefit.” Soon, brokers won’t even have to leave the office – they’ll be able to conduct meetings through the windows.

With all these projects coming at once (bringing over half a million sq m of floorspace) Richards realises clearer guiding principles must finally be set out. For the first time, he has begun to model what all the City’s constraints actually look like in 3D, developing a kind of “jelly mould” for all future developments. He began by drawing a mountain-shaped dome over the cluster, then went around the distant viewing points one by one, “chipping away at the foothills”.

Within this general lumpy mould – which has the look of a mauled blancmange from some angles – he has started slicing away more specific areas. He talks of “ski slopes” swooping down towards St Paul’s and the Tower of London, how buildings must “ramp up” to the peak of the cluster, and how Gotham City is the “bookend” to the east, beyond which it’s unlikely more towers will be allowed (within the City limits at least).

Perhaps the tortured heap of towers that seem to be the future of London’s skyline (some thrilling, some monstrous, all very large) is inevitable. It is a vertical expression of the Square Mile’s medieval street pattern, forced skywards by global finance and massaged by reactive planning – the chaotic cocktail of invisible forces shaping the city.

Sources: 3D Model of One Undershaft provided by Eric Parry Architects, 3D model of London by Vertex Modelling, 3D model of London based on UKMap data provided by The GeoInformation Group, planning applications submitted to the City of London local authority, Greater London Authority